If adjectives come before the noun [excellent, bon etc.], do you use de [d'] or des?

Moderator note: Multiple threads have been merged to create this one. See also this thread discussing exceptions to the standard rule. If you are interested in the article to use before singular nouns, see this thread.

Thank you pheelineerie for that clarification. Well I thought that strictly speaking it would be "de nouveaux vetements", but not really learning grammar in a technical sense, I was unsure. Furthermore, when speaking and always saying "des" anyway (I think for ease and flow of speech), it increases the confusion.
Thanks also for providing the results It's hard to know the grammar rules strictly when you don't learn it as a foreign language

Edit: In response to pieanne, I know where you are coming from. I guess it's just for ease of speech, or better flow?

I entirely agree with pheelineerie. It's the way I've been taught and I had been a bit distressed to hear things like "des nouvelles industries". Fortunately I was really happy to see that it was not correct. I think it's just easier as there is a rule and no exception.

Still, I don't think it's that important when speaking. Lots of French speakers don't seem to bother with this rule!

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Agreed once more.
I guess "des jolies roses" is becoming very common but is not grammatically correct for the moment though it's commonly used.
I must say that since I've read the rule once again I feel better using "de", knowing that I'm right.
Here it what I also found in my Bescherelle.
At least it's easier for foreigners and natives as well as ears have nothing to do here ...

You can say both. "de" or "des" are grammatically correct in this case.
In general, you can use "de" when it's plural if the adjective is before the name, like
"on mange de bons fruits en été", or "il a de grandes oreilles",
but not in any case, I believe there is no grammar rule about it...
best

If the partitive particle de is followed by adjective, even if it is in plural form, it should always be de and not des. However, if the adjective that follows starts with a vowel, the de should be contracted to d'.

Not only the purists do , "des bonnes lignes" isn't totally incorrect but probably used in limited contexts. IMHO, I would rather say "de belles lignes", what are you talking about?

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I also say de belles lignes, but I think that based on the rule as described in grammar books these days, the usage is changing and that many people consider des (as well as de, of course) to be perfectly correct in front of plural adjectives that precede the noun. I probably misused the term "purists".

I understand what happens. What interests me is why, whether there is a history behind it.

Is there any reason why "J'ai des fleurs" keeps its indefinite article when we postpone the adjective ("J'ai des fleurs rouges") but loses it once we decide to place an adjective in front ("J'ai de jolies fleurs")?

I imagine that, for the Académie to authorise the change, there must be some logic or history behind it.

Or is it just one of those quirks that we have to learn, and which just happens "because"?

I'm afraid that I can't give you a reason, but I can tell you that the "rule" has nothing to do with postponing adjectives. The rule is that des > de before a prenominal adjective:
J'ai des fleurs.
J'ai des fleurs rouges.
J'ai de jolies fleurs.
J'ai de jolies fleurs rouges.

If adjectives come before the noun [excellent, bon etc.], do you use de [d'] or des?

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I am quite amazed that there are so many answers when we actually do not have any context or phrase.
Depending on the situation we could have (for example)
des belles fleurs
de bien belles fleurs
les belles fleurs que l'on m'a offertes

OK, a few days after (re-)starting this thread, I can answer my own question, thanks to this piece of academic research on the subject "Opposition Entre De Et Des Devant Les Noms Précédés D'Epithète En Français." (Thanks to itka for the link.)

I’ll summarise it here for others’ benefit, since it’s not the easiest thing to read as it’s written.

From the paper​

The researchers use 14,500 examples of the construction, taken from corpora dating from 1601 to 1986.

They confirm that both forms are used and suggest that it is linguistic "weight" which determines which of the two forms that people will choose. "Weight" could be simplified as whether an adjective is heavy or light, lighter adjectives being associated with 'des'.

Going back to the introduction of the rule, they confirm that 'de' is used in 83.7% of the cases. However, this frequency in the sixteenth century seems to have depended on the location of the writer, the sample showing a southern writer occasionally using 'des' (in 14% of cases) whilst a northern writer uses 'de' exclusively.

After the eighteenth century there is no longer any variation that can be attributed to location, usage being uniform in all areas of France.

The sample now concerns exclusively the later half of the twentieth century. The researchers list four factors which determine which word one would choose.

First factor: The level of language to be used
As we may suppose ourselves, usage of 'des' springs up more readily when the text in question is less official than, say, treaties and essays. The peak is with online discussion forums like this one, where 'des' is used in around one third of instances.

In the more formal writing, 'de' is nearly always preferred. Where 'des' is used, the researchers note that it accompanies specific adjectives that one might call 'light'. For example, they note that there are 617 uses of 'de' in academic texts against 30 of 'des'. Of these 30, 23 occur when the adjective is petit(e)s. Petit is considered to be a 'light' adjective, and its presence caused the academic authors to deviate from their customary 'de' (used in 95% of cases) to 'des'.

Interestingly, there is one scenario in which 'des' greatly dominates. It is used in about 85% of cases of spoken French in the sample.

Second factor: Liaison
To get around contamination the researchers look only at samples which come from treaties.

There were only 30 cases of 'des' being used and they noticed that 29 of them occurred where the noun concerned started with a consonant (the exception being des grands z-auteurs). In other words, there was no liaison. If words had a liaison, 'de' was always used, except in that one exception.

Their theory is that liaison causes the word to seem bigger, lengthier, 'heavier’. As is usual with their findings, the lighter epithets take ‘des’ and the heavier ‘de’. Without liaison ‘de’ was used 86% of the time. Words with liaison (so heavier) saw ‘de’ used 92% of the time.

Third factor: AccentuationOnce an adjective is given more weight, uses of ‘des’ switch to ‘de’, in line with the central theory of this paper.

There are two ways of increasing the weight of an adjective. One can either repeat it (“Je te fais de gros gros bisous”) or use a modifier (“Il a pris de très mauvaises habitudes”).

Fourth factor: Grammatical genderThe samples were tailored to correspond only to abstract nouns, in what the researchers called a bid to “éliminer autant que possible des facteurs sémantiques”.

Masculine adjectives can be shorter than their feminine equivalents. In every sample that they looked at (newspapers, novels, magazines, online forums), masculine nouns stood a higher chance of being associated with ‘des’. Of the sample 22% of masculine adjectives were preceded by ‘des’, compared to 8% of feminine forms. Again, the procedure seems to be that we unknowingly opt for ‘des’ before lighter adjectives, and masculine adjectives are considered to be lighter than their feminine equivalents (since feminine forms often follow the formula “masculine adjective + some bit of information”).

Tim’s overall summary
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So the “rule” (optional though it may be) has been in place for several centuries. At higher levels of French ‘de’ dominates. As the formality of the work decreases, the prevalence of ‘des’ increases. Nonetheless ‘de’ is still the more regularly used form. The only exception is in speech, in which case ‘des’ is used in about 85% of cases. (In this respect ‘des’ seems to map the English ‘gonna’.)

Even at formal levels ‘des’ makes an appearance with certain adjectives, just because they are so light linguistically that it’s hard to curtail the instinct of doing applying 'des' to them.

Light adjectives which would normally be fronted with ‘des’ can become heavier in a number of ways.

1) When there is liaison between the adjective and the noun that it describes, the adjective becomes something heavier, owing to the increase in length.

2) When an adjective is doubled or modified by an adverb, its linguistic weight is increased and popular usage turns what may have been ‘des’ into ‘de’.

3) Finally an adjective that we subconsciously treat as light in its masculine form and to which we accord ‘des’ may fall under the heavier category when we use it in its feminine form. Samples show that it is nearly three times as common to apply ‘des’ to the masculine adjective as it is to the feminine.

Second factor: Liaison
To get around contamination the researchers look only at samples which come from treatiestreatises/essays.

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The effect that the authors observed is very minor, but still surprising. But a much more significant factor (which they did not study, because it doesn't support their weight hypothesis) is liaison/elision between d(e)/des and the immediately following word. For example (with a quick Google search) :